1. Field of the Invention
This invention relates to a method and apparatus for controlling the temperature of a semiconductor wafer in a semiconductor manufacturing apparatus, and particularly to a wafer temperature control method and a wafer temperature control device which in a manufacturing process carried out inside a semiconductor manufacturing apparatus such as an etching apparatus for processing a wafer at a low temperature keep the temperature of the wafer at a set temperature with high accuracy and improve responsiveness of the temperature of the wafer to changes in the set temperature.
2. Description of the Related Art
In present semiconductor technology, improvement of manufacturing process technology is indispensable for the realization of greater integration, greater density of integration and higher performance of devices. In particular, in a dry etching apparatus used for a fine manufacturing process, to carry out highly anisotropic etching, control maintaining the temperature of a wafer at an extremely low temperature (-70.degree. C. or below) with high accuracy during the process is necessary. That is, it is necessary to carry out etching in an extremely low temperature state in order to control etching rate and selection ratio with high accuracy. Also, as semiconductor devices are made smaller, pattern densities and contact hole aspect ratios become larger, and it is necessary to achieve increased etching accuracy by carrying out highly anisotropic etching. An extremely low temperature state is necessary to achieve this high anisotropy.
As an example of extremely low temperature etching, the content of a presentation given by K. Tsujimoto et al. in 1988 is included in Proceedings of Symposium on Dry Processes (Oct. 24-25, 1988, Tokyo), pp. 42-49.
However, with this method, because the temperature controllability is poor, returning a wafer having been brought to a low temperature to room temperature takes a long time. Consequently, using an apparatus based on this method for production has been problematic. Furthermore, the method also left unresolved problems of set temperature stability.
An apparatus conceived to solve this problem is shown in FIG. 1.
In this apparatus, a wafer 70 is placed on a lower electrode 63 inside a vacuum chamber 61 and a manufacturing process is carried out in a plasma atmosphere while the wafer 70 is cooled by way of the lower electrode 63 by liquid nitrogen or freon gas or the like sent from a refrigerating machine 64 through a pipe 71. In this case, temperature control keeping the wafer 70 at an extremely low temperature is carried out by a signal from a sensor 66 detecting the temperature of the lower electrode 63 being inputted into a control box 72 and the opening angles of a main valve 51 and a bypass valve 52 disposed in the pipe 71 being controlled with commands from the control box 72 and the flow quantity of the coolant thereby being controlled.
However, because temperature control of the coolant itself is difficult, wafer temperature control in the related art semiconductor manufacturing apparatus described above has been performed by means of feedback control of the coolant flowrate, and when the temperature of the wafer 70 has fallen below a set temperature (target value) at a time of plasma stoppage or due to an outside disturbance it has taken time to return to the set temperature. Furthermore, the responsiveness to changes in the set temperature has also been bad. Explaining this more specifically, related art wafer temperature control has been carried out just by controlling the coolant flowrate, and no heat source has been provided. Therefore, as shown in FIG. 3, although when the wafer has risen in temperature when a plasma is impressed it has been possible to suppress the temperature rise to a certain extent by increasing the opening angle of the main valve 51 and thereby increasing the flowrate of coolant to the lower electrode 63, when the wafer has fallen in temperature at a time of plasma stoppage or due to an outside disturbance the only method of returning it to the set temperature has been to close the main valve 51 and open the bypass valve 52 and stop the cooling of the wafer 70 on the electrode by the coolant. Consequently, it has taken time for the wafer to return to the set temperature which is the target value.
Also, during a process the temperature of the wafer 70 is reduced to an extremely low temperature of about -100.degree. C. and furthermore, because the inside of the vacuum chamber 61 is a vacuum, once the wafer 70 is reduced to an extremely low temperature this temperature is maintained by vacuum insulation. Consequently, over half a day has been required for the operation of returning the wafer from -100.degree. C. to room temperature after the completion of wafer processing.
Because the stability of the wafer temperature and the responsiveness to changes in the set temperature of related art wafer temperature control have thus been poor, satisfactory control of the above-mentioned high anisotropy of etching has not been achieved and consequently it has not been possible to perform a fine etching process with high accuracy and it has not been possible to obtain a high quality product.